Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the image receiving surface. In these solid ink printers, also known as phase change inkjet printers, the solid ink can be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. Once the melted solid ink or the gel ink is ejected onto the image receiving member, the ink returns to a solid, but malleable form, in the case of melted solid ink, and to a gelatinous state, in the case of gel ink.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving surface to form an ink image during printing. The image receiving surface can be the surface of a continuous web of recording media, a series of media sheets, or the surface of an image receiving member, which can be a rotating print drum or endless belt. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an aperture, usually called a nozzle, in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The magnitude, or voltage level, of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data. A print engine in an inkjet printer processes the image data to identify the inkjets in the printheads of the printer that are operated to eject a pattern of ink drops at particular locations on the image receiving surface to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
Phase change inkjet printers form images using either a direct or an offset print process. In a direct print process, melted ink is jetted directly onto recording media to form images. In an offset print process, also referred to as an indirect print process, melted ink is jetted onto a surface of a rotating member such as the surface of a rotating drum, belt, or band. Recording media are moved proximate the surface of the rotating member in synchronization with the ink images formed on the surface. The recording media are then pressed against the surface of the rotating member as the media passes through a nip formed between the rotating member and a transfix roller. The ink images are transferred and affixed to the recording media by the pressure in the nip. This process of transferring an image to the media is known as a “transfix” process.
Offset phase change inkjet printers utilize drum maintenance units (DMUs) to facilitate the transfer of ink images to the recording media. A DMU is usually equipped with a reservoir that contains a fixed supply of release agent (e.g., silicone oil), and an applicator for delivering the release agent from the reservoir to the surface of the rotating member. One or more elastomeric metering blades are also used to meter the release agent onto the transfer surface at a desired thickness and to divert excess release agent and un-transferred ink pixels to a reclaim area of the drum maintenance system. The collected release agent is filtered and returned to the reservoir for reuse.
During transfer of the ink images from the image receiving member to the recording media, the recording media extracts a small quantity of release agent from the surface of the image receiving member. The ink pixels that are placed on the oiled and metered imaging surface typically remove more oil from the imaging surface during transfer than the non-inked media does. This difference in oil removal produces a differential oil “ghost” of the image that resides on the image receiving member until the next drum maintenance cycle. This oil “ghost” is erased by the subsequent flooding of oil on the imaging member by the oil applicator and the metering of the release agent layer by the elastomeric blade. Additionally, since some ink drops in an ink image often fail to transfer to the recording media, the image receiving member carries some residual ink drops that can transfer to a subsequent recording media resulting in a “freckling” effect on the subsequent recording media. To reduce or prevent ghosting and freckling, the DMU applies a new coating of release agent to the image receiving member after each image formation and transfer operation from the image receiving member. The metering blade removes residual ink drops from the image receiving member and forms a layer of release agent with uniform thickness on the image receiving member. Frequent use of the DMU reduces the operational life of the DMU, but is necessary in existing printers to avoid excessive ghosting and freckling.
Indirect inkjet printers are capable of producing either simplex or duplex prints. Simplex printing refers to production of an image on only one side of a print media. Duplex printing produces an image on each side of a media sheet. In duplex indirect printing, an ink image is initially formed on an intermediate drum and then transferred to the media. The media sheet is then inverted and sent along a path that passes the second side of the media sheet by the intermediate drum upon which the ink has been deposited for the formation of a second ink image on the second side.
In indirect printing systems, significant levels of oil on the media before imaging is undesirable since the release agent can prevent ink from properly adhering or transferring to the media. Therefore, preventing the release agent from transferring to the back side of a sheet during printing of the first side image is desirable. Current printing systems must slow down and use special sequencing in duplex mode to prevent release agent from being transferred to the back of a sheet during front side printing. In an indirect printer, if the transfix roller and intermediate member also contact one another before a media sheet reaches the transfix nip or between sheets as they pass through the nip and release agent transfers to the transfix roller from the intermediate member.
For duplex prints, the excessive release agent transferred to the second side of the media sheet can interfere with the printing of ink images on the second side of the media sheet. In an indirect printing system, the presence of release agent on the media can result in some ink remaining on the intermediate member instead of transferring to the media. In both cases, the loss of ink produces an image having partial or missing pixels. When the partial or missing pixels are detectable by the human eye the phenomenon is known as image dropout.
In some areas on the surface of the drum where no recording media is placed, the drum can contact the transfix roll and oil can be transferred from the drum to the transfix roll. This oil can then be placed on the backside of subsequent sheets of recording media transported by the drum if printing in a duplex mode, where the backside recording media receives an image after the frontside of the recording media. If oil has been place on the backside of the sheet before printing, the transfer of the image to the backside of the sheet can be of poor quality, causing an image quality defect. The presence of release agent can be a problem especially in the “interpanel” zones or spaces between the second end of one sheet and the first end of a second sheet being sequentially printed. The improper deposit of oil on a recording sheet can also occur where print jobs are made with mixed paper sizes, especially in the cross-process direction. Thus, improvements to printers to reduce or eliminate the application of or amount of release agent to unwanted areas of the imaging drum and the transfix roller during printing are desirable.